What are the reasons for unstable motorcycle idle speed, sometimes high and sometimes low?
2 Answers
The reasons for unstable motorcycle idle speed include carbon deposits in the throttle body and low fuel pressure. Carbon deposits in the throttle body: When the engine oil temperature rises, it generates some oil vapor. The oil vapor passes through the throttle body before entering the cylinder. When oil vapor is present in the throttle body, it easily attracts dust. The air filter cannot completely filter all the dust, so a small amount of dust passes through the throttle body, gradually forming carbon deposits over time. To address this, remove the intake hose to expose the throttle body, disconnect the negative terminal of the battery, and turn off the ignition switch. Low fuel pressure: This can be caused by internal blockage of the high-pressure fuel pump, abnormal operation of the pressure regulator, the cold start solenoid valve stuck in the normally open position, the fuel injector spraying fuel near the intake valve, damage to the fuel injector causing excessive return fuel flow, or a large amount of fuel leaking into the cylinder. Additionally, bends, collapses, or blockages in the low-pressure fuel line can cause poor fuel supply. To check this, consider loosening the outlet of the fine filter and using the starter to crank the engine.
I've encountered the issue of unstable motorcycle idle speed, fluctuating up and down, many times. There are several main reasons for this. A dirty or clogged carburetor is the primary culprit, causing uneven fuel supply and erratic engine RPM. An air filter that hasn't been replaced for too long and is clogged with dust can also restrict air intake, affecting the air-fuel mixture ratio. If the spark plug has excessive carbon buildup or is worn out, the ignition becomes unstable, naturally causing idle speed to fluctuate. Aging vacuum lines with leaks can alter vacuum pressure, similarly causing RPM fluctuations. Throttle body carbon deposits are another common factor, where sticky operation leads to instability. I recommend starting with the simplest solutions first - cleaning the carburetor and replacing the air filter. If that doesn't work, then check the spark plug and vacuum lines. Regular maintenance can prevent many issues; don't wait for small problems to become big ones.
